Field
The presently disclosed subject matter relates to a two-dimensional optical deflector. The optical deflector can be applied as an optical scanner to a laser, pico projector, a laser radar, a bar code reader, an area sensor, an adaptive driving beam (ADB) type head lamp, a head-up display (HUD) unit, and other optical apparatuses, to generate scanning light.
Description of the Related Art
Generally, in an optical scanner or the like, a two-dimensional optical deflector is constructed by a micro electro mechanical system (MEMS) device manufactured by using semiconductor manufacturing processes and micro machine technology.
A two-dimensional optical deflector can be constructed by simply combining two one-dimensional optical deflectors each with one mirror; however, in this case, since laser light is reflected twice, once by each mirror, the utilization of laser light is low and the size is large. Therefore, a two-dimensional optical deflector has been constructed by only one mirror, in order to increase the light utilization efficiency of laser beam and reduce the size.
A first prior art two-dimensional optical deflector with only one mirror has two of the same kinds of piezoelectric actuators (see: FIG. 19 of JP2008-40240A). In more detail, this two-dimensional optical deflector is constructed by a mirror, a pair of torsion bars coupled to the mirror along an axis (X-axis), an inner frame (movable frame) surrounding the mirror and the torsion bars, inner piezoelectric actuators coupled between the torsion bars and supported by the inner frame via inner coupling portions, serving as cantilevers for rocking the mirror with respect to the X-axis of the mirror, an outer frame (fixed frame) surrounding the inner frame, and outer piezoelectric actuators of a meandering-type coupled between the inner frame and the outer frame, serving as cantilevers for rocking the mirror along another axis (Y-axis) of the mirror.
In the above-described first prior art two-dimensional optical deflector, the inner piezoelectric actuators are driven by a drive voltage such as 10 V at a relatively high resonant frequency such as 25 kHz for a horizontal scanning, while the outer piezoelectric actuators are driven by a relatively high drive voltage such as 60 V at a relatively low non-resonant frequency such as 60 Hz for a vertical scanning. Thus, a piezoelectric driving method is used for both of the horizontal scanning and the vertical scanning.
In the above-described first prior art two-dimensional optical deflector; however, in order to enhance the resolution of projected images, the high frequency for a horizontal scanning has to be increased to 30 kHz or more, so that it is difficult to make the resonant frequency compatible with the non-resonant frequency. Also, in this case, in order to suppress the dynamic deformation of the mirror and the spurious vibration of the piezoelectric actuators, the mechanical rigidity of the optical deflector has to be increased. Further, the drive voltage for the vertical scanning is very high.
Particularly, when the outer piezoelectric actuators are of a meandering-type, the vibration mode of the inner piezoelectric actuators and the vibration mode of the outer piezoelectric actuators easily interact with each other, so that it is difficult to maintain the flexing angle of the mirror by the non-resonant frequency while suppressing the mode interaction. Further, the optimum thickness of the mirror is different from that of the meandering-type outer piezoelectric actuators.
A second prior art two-dimensional optical deflector with only one mirror has two different-kinds of actuators: piezoelectric actuators and electromagnetic actuators (see: JP2011-64928A & US2011/0063702A1). In more detail, this two-dimensional optical deflector is constructed by a movable body and a base for supporting the movable body. The movable body is constructed by a mirror, a pair of first torsion bars coupled to the mirror along an axis (X-axis), an inner frame (first movable frame) surrounding the mirror and the first torsion bars, an outer frame (second movable frame) surrounding the inner frame, piezoelectric actuators coupled between the inner frame and the outer frame, serving as cantilevers for rocking the mirror with respect to the X-axis of the mirror, a pair of second torsion bars coupled between. the outer frame and the base along another axis (Y-axis) of the mirror, and a coil provided on the rear surface of the outer frame. On the other hand, the base has a recess portion in which a permanent magnet surrounded by a pair of yokes is provided. The movable body is bonded to the base, so that the movable body can be rocked along the X-axis and the Y-axis of the mirror.
In the second prior art two-dimensional optical deflector, the piezoelectric actuators are driven by a drive voltage such as 10 V at a relatively high resonant frequency such as 25 kHz for a horizontal scanning. Thus, a piezoelectric driving method is used for the horizontal scanning. Contrary to this, when a current is supplied to the coil, Lorentz forces are generated between the current and a magnetic field generated between the yokes of the permanent magnet (see: FIG. 7 of JP2011-64928A & US2011/0063702A1), the mirror is rocked along the Y-axis. The current is driven by a relatively low drive voltage at a relatively low non-resonant frequency such as 60 Hz for a vertical scanning. Thus, an electromagnetic driving method is used for the vertical scanning. In this case, since the Lorentz forces are very large, the flexing angle of the mirror can be large even at the low non-resonant frequency.
In the above-described second prior art two-dimensional optical deflector; however, in order to increase the Lorentz forces, both of the outer frame on which the coil is formed and the permanent magnet have to be increased in size, Which would increase the optical deflector in size.